Mammalian odorant receptors (ORs) are key molecules in mammalian olfaction. Each olfactory sensory neuron (OSN) is thought to express only one allele of one OR. The mechanisms through which OSNs select an OR allele for expression and maintain the monoallelic expression are unknown. Based on one of the two main models currently accepted we hypothesize that expression of an OR suppresses expression of other ORs in the same OSN. To test this hypothesis, we propose: (i) to find when the commitment to express a particular OR occurs; (ii) to express heterologously an OR gene in uncommitted OSNs or their progenitors; and (iii) to determine the effects of OR overexpression on endogenous OR expression. The project will yield not only information supporting or contradicting current models of OR expression regulation, but also information relevant to research on axon targeting. The proposed project is a part of a larger group effort that utilizes viral vectors to investigate the development, functional organization, and mechanisms of ligand recognition of the olfactory system.